Some GitHub actions improvements

[mkniewallner]

• Added tests for changed code. Not applicable
• Updated documentation for changed code. Not applicable

Bunch of improvements on GitHub Actions workflows to:

• bump all actions/* actions versions
• define the default shell to use, to avoid duplication
• bootstrap Poetry through install.python-poetry.org rather than the deprecated install-poetry.py script

Also needed to update the name for PyPy from pypy3 to pypy-3.7, since https://github.com/actions/setup-python/releases/tag/v3.0.0 deprecated the usage of pypy3 keyword for the more explicit pypy-3.x pattern.
pypy3 used to correspond to pypy-3.6 and not pypy-3.7, but since Poetry dropped support for Python 3.6, I thought it may be better to test against PyPy 3.7.

Note: Since pypy3 is a required status check, the CI waits for this job to be run (which will not happen), but you can see the job passing here.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
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No Coverage information
No Duplication information

[neersighted]

@abn or @sdispater need to twiddle the required checks to allow merging this, but LGTM!

[abn]

@neersighted replaced pypy3 with pypy-3. 7.